drm/vmwgfx: Fix invalid reads in fence signaled events

In the Linux kernel, the following vulnerability has been resolved: drm/vmwgfx: Fix invalid reads in fence signaled events Correctly set the length of the drm_event to the size of the structure that's actually used. The length of the drm_event was set to the parent structure instead of to the drm_vmw_event_fence which is supposed to be read. drm_read uses the length parameter to copy the event to the user space thus resuling in oob reads.